The invention relates to a fuel injection pump for internal-combustion engines.
German Offenlegungsschrift No. 2,903,482 and German Patent Application No. P 36 12 942.9, disclose a fuel injection pump in which the fuel quantity delivered to an injection nozzle during the delivery stroke of the pump piston is injected into the cylinder of the internal-combustion engine during a single opening phase of the injection nozzle. The delivery period, and thus the injected fuel quantity, is determined by the closing and opening instants of the electric pilot valve. With closing of the pilot valve, the pump working space is closed off, so that the delivery pressure builds up with the pump piston stroke, and with opening of the pilot valve, the pump working space is connected to a relief space, which causes the pressure in the pump working space to drop abruptly, which closes the injection nozzle and thus the injection is ended.
In the case of diesel engines, this so-called direct injection causes considerable noise problems, in particular during idling and at low speeds, due to the abrupt combustion of the injected mixture. A notable reduction in noise is achieved however by dividing the fuel injection quantity into a smaller preliminary injection quantity and a main injection quantity, dependent on the speed and load of the internal-combustion engine, as is known for example from German Offenlegungsschrift No. 3,444,234 or German Offenlegungsschrift No. 3,511,492.
In German Offenlegungsschrift No. 3,444,234, with a distributor fuel injection radial piston pump during the delivery stroke, turning of the distributor piston brings about a controlled opening of the relief opening, which connects the pump working space to an accumulator space of a predetermined volume. In this arrangement, during the delivery stroke, a constant delivery volume is branched off from the delivery quantity displaced to the injection nozzle. As a result, the delivery pressure in the pump working space drops to below the opening pressure of the injection nozzle and thereby closes the latter. The fuel preliminary injection is thus ended. With delivery pressure increasing again upon further delivery stroke of the pump piston, the injection nozzle opens again and the remaining fuel injection quantity is injected during the main injection phase. The main injection phase is ended as soon as the turning distributor piston opens a control opening leading to the fuel supply space.
According to German Offenlegungsschrift No. 3,511,492, in the case of a distributor fuel injection axial piston pump, an accumulator space with constant storage volume connected to the pump working space is relieved by an electric pilot valve during the delivery stroke of the pump piston. Due to the pressure drop brought about thereby in the pump working space, the injection nozzle is closed and the preliminary injection ended. The end of the main injection is fixed by the position of an annular slide valve displaceable over the pump piston, which valve opens a relief opening at a certain stroke position of the pump piston.
In the case of the two last-described known fuel injection pumps, in contrast to the first-described injection pumps, the overall injected fuel quantity is not fixed by an electric pilot valve, but mechanically by the rotational position of the distributor piston or by the axial position of the annular slide valve.